The Necromorphs' motivations are to create a mass of necrotic flesh which can then be collected by the Markers during Convergence with the intention of creating another Brethren Moon, perpetuating their galaxy-wide cycle of extinction. The Necromorphs are controlled via the Markers by a group of entities known as the Brethren Moons, which are massive, highly intelligent beings created after a Convergence Event is initiated by a Marker. They are believed by some to be the heralds of humanity's ascension, but on a more practical level, they are the extremely dangerous result of exposure to the enigmatic devices known as the Markers. The sole purpose of all Necromorphs is to acquire more bodies to convert and spread the infection. The resulting creatures are extremely aggressive and will attack any uninfected organism on sight. It didn't know how best to make a scary killing machine so it just went for 'Nazi Zombie'.Necromorphs are mutated corpses reshaped into horrific new forms by a recombinant extraterrestrial infection derived from a genetic code etched into and transmitted by the Markers. I would have assumed that Shamblers were the initial attempt by the Marker/Moon to 'interact' (consume) humans. Cos remember, stalkers are two corpses put together - same with leapers on Tau Volantis. What makes the Marker/Moon thing, 'I need a Brute/Tripod/Stalker'. Similarly with all those 'amalgamated' monstrosities. That lends to your theory of the outbreak having templates to draw from. No human babies on or orbiting Tau Volantis, so what does the Marker/Moon use? Dogs.

That said, the Lurkers from 1 and 2 are clearly human babies. Obviously maybe the Marker chooses what it needs rather than just rolls the dice on what 'breed' of Necromorph you get. Pukers and the Pack are just examples of this. Something both to cause fear and be better suited for killing the same species in a given environment. I think it was mentioned somewhere that the Necromorph shapes are just the Marker making the best murder weapon it can out of the body it can.

Dead space types of slasher